In the precise measurement of the weight of items, it is a requirement to minimize as far as possible all factors that have an influence on the weighing result. To name examples, air movements can in most cases be reduced by a draft protection device to such an extent that they become negligible, or fluctuations of the ambient conditions such as temperature or humidity can be eliminated for example by placing the balance in a climate-controlled room.
As a further factor which likewise has an influence on the weighing result, electrostatic charges which are present in the weighing compartment or in the environment or directly on the load to be weighed manifest themselves in particular when weighing electrically non-conductive samples or when weighing samples in containers consisting of a non-conductive material. The influence of electrostatic charges becomes a problem in particular when large polymer- or glass containers are used in a situation where the purpose of a weighing process is to determine small mass differentials. The weighing pan itself on which the samples and/or containers, i.e. the weighing loads, are placed is in most cases not accumulating an electrostatic charge because as a rule the pan is made of metal, and the charges can drain away through a conductive connection to the balance housing.
An accumulation of electrostatic charges on a load resting on the weighing pan causes a difference between the respective electrostatic potentials of the load and the grounded parts of the balance. Because of this difference in potential, there is a force acting between the load and, e.g., the floor of the weighing compartment. The vertical component of this force is therefore also acting on the weighing cell, in addition to the force generated by the load. This causes an error in the weighing result that is delivered and displayed, which is undesirable in particular in balances of high resolution.
Other parts of the balance such as for example a draft protection device, if there is one, or other parts of the housing to the extent that they consist of a non-conductive material, can likewise accumulate an electrostatic charge and cause errors in the weighing result through a so-called parasite force pattern.
An analytical balance for weighing electrostatically charged objects is described in U.S. Pat. No. 6,515,238 B1. The balance, which is equipped with a draft protection device, has a device for generating an ionized air stream, which includes an ionization device with at least two point electrodes, each of which carries a voltage of opposite polarity relative to its respective counterpart electrode, and which also includes a fan. The device can be activated by the closing of the access openings of the draft protection device and deactivated after a predetermined running time.
The device just described has the disadvantage that on the one hand the fan can produce undesirable air currents and on the other hand there is always a given fixed amount of ionization charges available for neutralizing the electrostatic charges, independent of the magnitude of the electrostatic charge accumulated by the load on the weighing pan.
An unmet advantage of the prior art is to provide a method and a device which make it possible to operate a balance in particular in the presence of electrostatic charges in accordance with the requirements of the user.